1. Field of the Invention
The present invention relates generally to an apparatus for extending the battery life in a mobile communication system. More particularly, the present invention relates to a power amplifying apparatus for a Wideband-Code Division Multiple Access (W-CDMA) mobile phone.
2. Description of the Related Art
In order to save the battery life, a power control scheme is used in that the distance between a mobile phone and a base station is estimated so that the mobile phone can adjust its transmitting power to achieve at least minimum required E/I at the receiver. To this end, the mobile phone monitors its reception power to determine how much transmission power is to be adjusted. FIG. 3 illustrates a conventional structure of a power amplifying apparatus of a mobile phone. In FIG. 3, a high power amplifier (HPA) 21 amplifies the power of an input signal under the control of an automatic power controller (APC) 23 and feeds the amplified signal to a transmission antenna 24. A power monitor 22 serves to monitor the transmission power received from the HPA 21. The APC 23 controls the transmission power of the HPA 21 according to the output information received from a digital signal processor (DSP: not shown) of a phone, or from the output information from a controlling Central Processing Unit (CPU) (not shown) and the output monitor 22. The output information comprises a DC signal, which is produced from a digital signal output from the DSP or the CPU via a digital-to-analog conversion (DAC) process. Then, the DC signal is used to control the transmitting power of the mobile phone.
The power added efficiency (PAE) of the HPA 21 is used to determine power consumption efficiency, thus it is an important factor in determining the life span of a battery. FIGS. 4 and 5 illustrate graphs showing the relationship between the input/output power characteristics and the PAE. For example, FIG. 4 shows the relative power consumption information when a GaAs-FET is used as the HPA 21. In such a case, the maximum PAE is obtained in the vicinity of the peak transmission power, as shown in FIG. 4. On an average usage of a mobile phone, a majority of the time is passed in a standby mode requiring less power consumption, thereby extending the battery life time. However, when voice transmission is required from the time to time, the peak transmission power, depending on the transmission environment, is required.
In view of foregoing in recent years, various suggestions have been made to extend the life span of the battery by adjusting the bias current and gate-source voltage. As shown in FIG. 5, a DC bias current flowing through the HPA 21 is adjusted based on the output of the HPA 21. Curves P1, P2, and P3 correspond to the output power with respect to E1, E2, and E3 (PAE). P1 and E1 represent power conversion efficiency for a relatively low DC bias current, whereas P3 and E3 represent for a relatively high DC bias current.
In recent years, there has been an increasing tendency of employing power amplifiers using a GaAs-FET, a transistor having high power conversion efficiency. With reference to FIG. 6, static characteristics differ in GaAs-FETs caused by changes in the manufacturing process. That is, the static characteristics, such as pinch-off voltage and drain saturated current, are greatly changed due to the manufacturing differences. To control the drain current (Id) as shown in FIG. 6 to a drain current of a design value, the individual difference in the static characteristics of different amplification transistors should be minimized. However, this is impossible in the current situation. Moreover, to obtain an ideal amplification transistors having a desired characteristic for the operation of amplifier, the manufacturing cost has to increased significantly because a large volume of transistors has to be produced to achieve few transistors exhibiting the ideal static characteristics.
It is, therefore, one object of the present invention is to provide an amplifier that resolves the above shortcomings in such way that the PAE of the power amplifier during a standby mode is increased.
It is, therefore, an object of the present invention to provide a power amplifying apparatus for a mobile phone which can increase the life span of a battery in a normal use state, by precisely controlling the DC bias current that determines the PAE of the power amplifier.
The above object can be achieved by providing a power amplifying apparatus, which uses a transistor as a power amplifying device, wherein a non-volatile memory records the static characteristic measurements of the transistor. The DC bias current of the transistor is controlled according to transmission power of the power amplifier by referring to the pre-recorded static characteristic measurement data, by changing the gate voltage of the transistor or by changing a drain voltage of the transistor having the gate voltage fixed, or by changing both the gate voltage and the drain voltage.